Islamist extremists are believed to have made inroads in Syria as instability has spread since the start of the uprising against President Bashar Assad 14 months ago. However, many Syrians believe the car bombs this year are in fact set off by government agents to discredit the revolution.

The video said the bombs were a response to attacks on residential areas.

"We promised the regime in our last declaration to respond to its killing of families, women, children and old men in a number of Syrian provinces, and here we kept our promise," it says. It calls for the protection of Sunni Muslims and threatens revenge against Alawites, the Shiite offshoot to which President Assad and many members of his security services belong.

"We tell this regime: Stop your massacres against the Sunni people. If not, you will bear the sin of the Alawites. What is coming will be more calamitous, God willing," it says. It also advises Sunnis to avoid security offices - a veiled threat of future bombings.

Violence has dropped since a United Nations observer mission arrived a few weeks ago, although more than 9,000 people have been killed since the beginning of the uprising.

There were reports of new violence on Saturday, and Turkish officials said two Turkish journalists who were detained two months ago were on their way home.